In conventional free electron lasers (FEL), a beam of relativistic electrons passes through a static periodic magnetic field (wiggler) to amplify a coherent optical wave to produce a powerful beam of coherent light. The pondermotive potential set up by the optical wave and the wiggler field modulates the electron beam longitudinal density on the order of a wavelength of light to form electron bunches. Electron bunching only occurs effectively after the optical wave intensity has built up after several round trips between mirrors in the resonator. This necessitates precise alignment of the resonator mirrors to ensure temporal and spatial overlap between the electron beam pulses and the optical pulses. The precise alignment of the resonator mirrors is critical to the performance of the FEL. Even a small misalignment of the mirrors can disrupt operation of the FEL.
Because of this problem, many FELs employ very sophisticated and expensive mirror alignment procedures. In addition, the efficient extraction of the electron beam energy requires very high intracavity optical power, which itself repetitively subjects the resonator mirrors to excessive optical intensity. The resulting mirror damage often sets a limit on the peak power attainable with such FELs.
Another FEL method involves self-amplified spontaneous emission (SASE) of light. These FELs do not use resonator mirrors, and do not suffer from the problems outlined above. However, SASE requires a very bright electron beam (i.e. high peak current, low emittance and energy spread), and a very long undulator to build up the beam intensity from spontaneous noise to a saturated intensity. The use of a long undulator necessarily causes excessive optical vignetting loss, which requires the use of optical guiding. But, for optical guiding to be effective, the small-signal gain has to be very high and the undulator has to be fabricated to extreme precision. All of this places extremely stringent requirements on the electron beam quality and the undulator performance.
The present invention provides a new, and significantly improved technique for generating coherent radiation with electron beams traveling near the speed of light in a magnetic undulator, without the need for resonator mirrors or optical guiding.
Accordingly, it is an object of the present invention to provide a FEL that does not require resonator mirrors for optical intensity build-up.
It is another object of the present invention to provide a FEL that does not require optical guiding for optical beam propagation.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.